Disclaimer/Complaints regulations If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible.

The [111] longitudinal sound velocity ( v L ) in a single-crystal synthetic opal has been measured at a frequency of 10 MHz in the temperature range 4.2–300 K. At 300 K, v L =2.1×10 5 cm/s. The quantity dv L / v 300 K ( T ) (where v T ,K −v 300 K ) in the ranges 4.2–200 and 200–300 K behaves in the way typical of amorphous and crystalline solids, respectively.

We report results of the ultrasonic investigation of Ba1-xKxBiO3 single crystals for two potassium concentrations x approximate to 0.35 and x approximate to 0.47 in a wide temperature range. The softening of both the transverse c(44) and the longitudinal dr modes have been observed at temperatures between 200 K and 50 K. In the case of Ba0.65K0.35BiO3 a pronounced hysteresis was discovered. We proposed a model in which the softening of the elastic moduli, the hysteresis, and the maximum in the attenuation of sound can be explained by assuming a coupling of the acoustic modes with the anharmonic oscillations of O-6 octahedra.

For describing the first-order isostructural valence phase transition in mixed valence compounds we develop a new approach based on the lattice Anderson model. We take into account the Coulomb interaction between localized f and conduction band electrons and two mechanisms of electron-lattice coupling. One is related to the volume dependence of the hybridization. The other is related to local deformations produced by f- shell size fluctuations accompanying valence fluctuations. The large f -state degeneracy allows us to use the 1/N expansion method. Within the model we develop a mean-field theory for the first-order valence phase transition in YbInCu_4. It is shown that the Coulomb interaction enhances the exchange interaction between f and conduction band electron spins and is the driving force of the phase transition. A comparison between the theoretical calculations and experimental measurements of the valence change, susceptibility, specific heat, entropy, elastic constants and volume change in YbInCu_4 and YbAgCu_4 are presented, and a good quantitative agreement is found. On the basis of the model we describe the evolution from the first-order valence phase transition to the continuous transition into the heavy-fermion ground state in the series of compounds YbIn_1-xAg_xCu_4. The effect of pressure on physical properties of YbInCu_4 is studied and the H-T phase diagram is found.

It is shown how the history of the growth of an icosahedral Zn-Mg-Y single grain can be determined by measuring the yttrium distribution. The growth mechanism and the stabilization of the icosahedral Zn-Mg-Y, RE (RE = rare earth: Ho, Er, Dy, Gd, Tb) quasicrystals are discussed with respect to structural investigations on related crystalline phases. We also show results of optical and ultrasonic investigations on icosahedral Zn-Mg-Y single crystals. They fit well to the discussed growth and stabilization mechanism.

We report results of the ultrasonic investigation of Ba1−xKxBiO3 single crystals for two potassium concentrations x ≈ 0.35 and x ≈ 0.47 in a wide temperature range. The softening of both the transverse c44 and the longitudinal c11 modes have been observed at temperatures between 200 K and 50 K. In the case of Ba0.65K0.35BiO3 a pronounced hysteresis was discovered. We proposed a model in which the softening of the elastic moduli, the hysteresis, and the maximum in the attenuation of sound can be explained by assuming a coupling of the acoustic modes with the anharmonic oscillations of O6 octahedra.

Elastic properties of CeCo2 single crystals have been investigated. It has been found that CeCo2 shows a remarkable elastic softening in c44 and (c11–c12)/2. The elastic anomaly suggests a very narrow band of 149K. Acoustic quantum oscillation has been successfully observed in CeCo2. The measurement by using 3He–4He dilution refrigerator gave us the effective mass of κ-orbit. The quantum oscillation frequency and the value of the effective mass are 193T and 1.17m0, and 232T and 1.73m0 for the magnetic field directions parallel to [100] and [110], respectively.

Ultrasonic measurements were performed on a fci Zn-Mg-Y quasicrystal and on the related parent compound Zn2Mg. Absolute values of both the longitudinal and the transverse sound velocities as well as their temperature dependence were determined, The temperature dependence of the quasicrystal sound velocity in the millikelvin temperature range is very similar to the logarithmic behaviour observed for amorphous materials.

The crossing of the temperature dependences of sound velocity in the normal and the superconducting state of metallic glasses indicates renormalization of the intensity of sound interaction with two-level systems (TLS's) caused by their coupling with electrons. In this paper we examine different theoretical approaches to a quantitative description of the renormalization, using the results of a low-temperature ultrasonic investigation of Zr41.2Ti13.8Cu12.5Ni10Be22.5 amorphous alloy. It is shown that the adiabatic renormalization of the coherent tunneling amplitude can explain only part of the whole effect observed in the experiment. There exists another mechanism of the renormalization affecting only nearly symmetric TLS's, which may be associated with the effect of electron density fluctuations on the interwell potential.

We have carried out ultrasonic investigations of (TMTSF)(2)PF6 in the spin-density-wave (SDW) ground state. Some features for the elastic behavior of the SDW state are found. One of the acoustic modes shows a jump in the sound velocity at the critical temperature of the SDW transition (T(SDW)approximate to 12 K) and a relaxation peak in the attenuation of sound just below T-SDW From this peak we have estimated the characteristic relaxation time of the spin fluctuations along the b axis to be t(0) = 2 X 10(-11) a. The sound velocity of another acoustic mode changes with temperature as the square of the order parameter. Besides a maximum just below T-SDW the attenuation of this mode has an exponential behavior at low temperatures and yields an energy gap value equal to 2 Delta(0) approximate to 96 K, which is much larger than the BCS value 2 Delta(0)approximate to 43 K. In addition there is an acoustic mode which displays a slow decrease of the sound velocity in the SDW state. We have also studied the influence of a magnetic field applied along the ce axis on the acoustic properties of (TMTSF)(2)PF6: T-SDW increases in the magnetic field. We discuss our experimental results in the frame of theoretical considerations which are based on the assumption that phonons influence the exchange interaction between the spins of the conducting electrons and thus couple to the SDW (magnetoelastic coupling). In our model, the temperature behavior of the sound velocity and the attenuation of sound is related to the real and imaginary parts of the dynamical spin susceptibility. This model allows us to classify the temperature behavior of different acoustic modes and to explain the observed anomalies in the sound velocity and the attenuation of sound in the SDW ground state. [S0163-1829(99)06821-6].

We present an experimental investigation of the temperature and magnetic field dependence of the elastic constants in the two stannide compounds Yb3Rh4Sn13 and Ca3Rh4Sn13. We find a small elastic softening of the c44 mode of ca. 2 % in the normal state. The temperature dependence of the various elastic modes are quite anomalous in the superconducting state. A comparison of our results is given with corresponding results of CeRu2, an amorphous superconductor and other amorphous materials.

Acoustic measurements of (TMTSF)2PF6 have been performed in the temperature range 2 K < T < 40 K. The behavior of the sound velocity and attenuation observed at the transition to the SDW ground state 12 K depends on the particular mode propagating in the crystal: Mode I shows a jump of the sound velocity at tsdw; for mode II the velocity increases with decreasing temperature as the square of the order parameter; a third type exhibits only a kink in the temperature dependence of the sound velocity at tsdw. We apply various models and discuss the coupling of the SDW to the lattice.

We present an experimental investigation of the temperature and magnetic field dependence of the elastic constants in the two stannide compounds Yb 3 Rh 4 Sn 13 and Ca 3 Rh 4 Sn 13 . We find a small elastic softening of the c 44 mode of ca. 2% in the normal state. The temperature dependence of the various elastic modes are quite anomalous in the superconducting state. A comparison of our results is given with corresponding results of CeRu 2 , an amorphous superconductor and other amorphous materials.

The superconducting energy gap and the parameter η determining the intensity of electron scattering at two-level systems in amorphous ZrTiCuNiBe are determined from the results of measurements of sound attenuation. The mechanism of adiabatic renormalization of the amplitude of coherent tunneling is used for a quantitative description of the peculiarities of sound absorption in the vicinity of Tc.

The heavy fermion superconductor CeCu2Si2 shows interesting low temperature properties and a complex B–T phase diagram. Crystal growth is complicated due to the high vapour pressure of Cu and Si at the melting point (about 1540°C), the peritectic melting behaviour and a lack of crucible materials. We have grown single crystals of CeCu2Si2 from a levitated melt using the Nacken Kyropoulos technique. We present ultrasonic measurements, which show the low temperature properties of different CeCu2Si2 single crystals.